Change Initiator vs Change Implementer

Answer:
Change initiator is someone who starts i.e. initiates Change Management process. That could be your customer, Service Level Manager, you Continual Service Improvement manager (or some similar role), Problem Management process...etc. That depends how did you set your Change management process.
Change Implementer is the one who implements change. Usually, Release and Deployment Management process is responsible to implement changes. Of course, standard changes (i.e. pre-approved changes) are not meant by that. They will be implemented by your administrators, IT Operations team or someone similar (again, depending on the organizational setup).

These articles will help you:
"Elements of Change Management in ITIL" https://advisera.com/20000academy/blog/
"ITIL/ISO 20000 Request for Change – Your steering wheel throughout the change lifecycle" https://advisera.com/20000academy/blog/2015/09/01/itiliso-20000-request-for-change-your-steering-wheel-throughout-the-change-lifecycle/